The prior art is replete with transmissions which are interposed in a typical power train between, for example, an internal combustion engine and the driving wheels of an automobile. A transmission is needed in such an application because the inherent operating characteristics of the engine and the load differ. The power necessary to move automobiles from a resting position has to be delivered in the form of a high (starting) torque and low (zero, or almost zero) angular velocity. This requirement is almost directly contrary to the operating characteristics of an internal combustion engine which is able to deliver such required amounts of torque only at angular velocities substantially in excess of zero. Thus a transmission is necessary between engine and load to alter the torque-angular velocity product, i.e. power, from one of moderate torque and moderate angular velocity as delivered by the internal combustion engine to a product of high torque and low angular velocity suitable for starting the vehicle. Once the vehicle accelerates, the torque-angular velocity product demanded by the load changes to one of decreased torque and increased angular velocity which, in turn, but for the existence of the transmission, would speed up the engine and force it into an operating region of higher speed where, from a durability standpoint it does not want to operate and, from a power standpoint--now that the load has been accelerated--it does not need to operate. Accordingly, a transmission is used to alter the torque-angular velocity product demanded by the starting phase to a new, different torque-angular velocity product suitable to both the engine and the (now moving) load.
Typically, one class of prior art transmissions achieves the required alteration by interconnecting the engine and the load through selectively engageable gear trains each of which has a different, but fixed, ratio. Thus, for this type of prior art transmission, known as a "manual" transmission, a human operator selects a different ratio of the transmission each time the previously selected relationship between the engine and the load becomes inappropriate. Furthermore, each time that such a relationship is altered, the power flow between the engine and the load must be interrupted so that the appropriate gear selection is not made under power. Thus, temporarily at least, the engine undergoes a change from full, or partial, load to a temporary state of little or no load until it again resumes a new operating condition once the new gear train has been selected. Such repeated cyclings thus force the engine into temporary states of inefficiency, and demand considerable experience and skill on the part of the human operator, especially in those conditions where, for example, the differing fixed ratios of a transmission amount to more than five, as is the case in truck transmissions. Moreover, the attempt to match engine and load characteristics through a series of fixed ratios is an approximation, at best, because each ratio selected is operative only in a very limited range of rotational speeds, or angular velocities.
Thus, it is a primary object of this invention to provide a new and improved transmission for use between a prime mover and a load.
It is another object of this invention to provide a transmission which achieves the necessary changes in power conversion between a prime mover and a load continuously, rather than as a series of discrete, fixed steps and without requiring interruption of the power train between the engine and the load.
The prior art has attempted to deal with the problems of manual transmissions noted above by a transmission commonly known as an automatic transmission. These devices are characterized by the inclusion of a hydraulic element, such as a fluid clutch or hydraulic torque converter, in an attempt to serve several functions, both to minimize the temporary, and total, disconnection of the engine from the load required by a manual transmission and, in the case of the hydraulic torque converter, to also function as a torque multiplication device prior to applying the engine power to a gear train contained within the automatic transmission. However, these hydraulic elements inserted into the power train between the engine and the load to minimize the disadvantages of another prior art device known as a clutch, and to thus simplify the operation of these types of devices from a human standpoint, suffer from the primary disadvantage that energy transfer between engine and load now occurs through the flow characteristics of a hydraulic fluid. This makes the power transmission less efficient because a portion of the energy transmitted is lost in the slippage characteristics of the hydraulic fluid. This has led, in the case of so-called automatic transmissions for automobile passenger car use, to the provision of a so-called "lock-up" torque converter, which eliminates the hydraulic elements--with their attendant disadvantages--from the power train and substitutes a direct mechanical connection between the engine and the load under certain operating conditions to increase fuel economy. Moreover, it is noteworthy that the types of transmissions employing hydraulic elements, and known partially as "automatic" for having eliminated the human-operable clutch from a transmission, still apply the power transmitted to a series of automatically selectable and selected gear trains. Typically, most transmissions of the "automatic" type employ a unique set of gears, known generally as planetary gears, selected portions of which are then brought into engagement--or disengagement--by the use of friction clutches or brake bands. While the engagement, or disengagement, of certain gear train members occurs, more or less, without human intervention, i.e., automatically, and thereby eases the task of operating the transmission for a human operator, this artifice still suffers from all of the deficiencies of a manual transmission, in that only a limited number of "fixed" gear ratios are provided. Moreover, the gains achieved in terms of human operability have been achieved at the cost of decreased efficiency, not only because of the provision of the hydraulic elements, but also because of the frictional losses involved when different gear train members of the automatic transmission are brought to a halt--by a clutch or the application of a brake band--to configure a new power path through, and hence different conversion ratio for, the transmission.
Accordingly, it is another primary object of this invention to provide a transmission which eliminates the need for hydraulic power transfer elements and motion decelerating devices such as friction clutches or brake bands.
It is still another object of this invention to provide a transmission in which all power is transmitted over and through mechanical devices such as gears, or the like.
It is still another object of this inention to provide a transmission the power conversion ratio of which is variable continuously and in infinitely variable incremental amounts.
In an attempt to provide the wide range of ratios required in a typical automotive application, without the bulk, complexity, cost, and operator inconvenience created by the provision of a large number of fixed ratios, the prior art has developed so-called CVTs (Continuously Variable Transmission) in which power is transmitted over belts and pulleys, the diameters of the latter of which are varied continuously over a given range to establish a continuously varying ratio between the driving and the driven pulley, thus effectively altering the transmission ratio of this type of device. While the required ratio changes thus occur continuously and variably, and thereby achieve a better match between the operating characteristics of an internal combustion engine and the demands of the load, the chief disadvantage of this type of transmission is the limited power, or torque, transmitting capability of the belt, as well as durability of the belts. This has limited application of these devices to low power requirements such as automotive passenger car applications in which engine size is below 2 liters of engine displacement. Power transmission over a belt and pulley introduces, particularly in higher power applications, frictional losses due to the slippage of the belt which is analogous to the slippage characteristics, and hence slippage losses, of the hydraulic elements provided in current automatic transmissions of the type described above.
Accordingly, it is still another object of this invention to provide a continuously variable transmission capable of transmitting powers and torques considerably in excess of the powers and torques transmittable over belts and pulleys.
Where continuous variability of the conversion ratio of a transmission is an absolute necessity, such as for example, in the operation of a highway paving roller moving at very slow speeds, the prior art has developed a continuously variable transmission known as a hydrostatic transmission. Generally, these types of transmissions employ hydraulic pumps which vary the pressure, and/or the amount of, oil supplied to hydraulic motors which are then connected to the load. However, the objective of continuous variability has been achieved in these transmissions at the cost of all the disadvantages of power transmission through a hydraulic fluid noted above and at the cost of bulky and expensive components required to deal with the high operating pressures of these devices.
Accordingly, it is still another object of this invention to provide a continuously variable transmission having a higher efficiency than the so-called hydrostatic transmission.
The preceding discussion of the prior art has concentrated primarily on a description of a transmission apart from its companion device, the so-called clutch. In the so-called "manual" transmissions previously described, the clutch functions primarily as a speed-equalization device to overcome the speed differences between a driving member and a driven member, which are greatest when the driven member is accelerated from a zero speed state. In addition, in this environment the clutch also functions to interrupt the power flow when a new gearset, with new speed ratios, is engaged for the purposes and in the manner previously discussed. It is noteworthy that in these types of transmissions, there is always a fixed ratio between the rotational speed of the driving member and the rotational speed of the driven member, as established by a gear ratio in operation at each particular time. In other words, the input speed to the transmission is directly related to the output speed of the transmission and the two speeds can be said to be kinematically dependent.
Those transmissions which employ a hydraulic coupling, or torque converter, in lieu of the friction clutches employed in a so-called "manual" transmission allow the input speed of the transmission and the output speed to be somewhat independent of one another because of the slippage in the fluid coupling. Still, even with the provision of a hydraulic coupling element, total independence of the input speed from the output speed of the transmission cannot be achieved, as the remainder of the transmission is still of the fixed ratio type and the slippage between the driving and the driven member of the fluid coupling can not be made too large to retain a nominal transmission efficiency.
Even the so-called CVTs discussed above need a clutch to overcome the large speed difference to be bridged under starting conditions which gap cannot be encompassed by the adjustment range of the variable diameter pulleys. Of all the prior art transmissions described above, only the hydrostatic transmissions are able to completely divorce the output speed from the input speed of the transmission, i.e., create a total kinematic independence between the speed of the prime mover and the speed of the load, long considered an ideal by those skilled in the art. The hydrostatic transmission achieves this goal, however, only at the cost and expense of the heat, and pumping, losses involved in the transmission of power through a hydraulic medium.
Accordingly, it is still another object of the invention to provide a transmission in which there is total kinematic independence between the input speed and the output speed thereof without power transfer through a hydraulic medium thereby also eliminating the need for a clutch inserted in the power train between engine and load.
The limitations in prior art transmissions noted above also have their effect on engine design in that they have forced engine designers to design internal combustion engines to operate over a wide range of rotational speeds and torque generating capability, although typically each internal combustion engine design provides only a very narrow range of rotational speeds at which its principal operating characteristics--torque generated and mimimum specific fuel consumption--are each at their optimum level for maximum efficiency.
Accordingly, it is another object of the invention to provide a transmission which allows engine designers to design engines which can operate within narrower operating ranges, thereby contributing to higher efficiency and lower fuel consumption rates.
One of the significant operating consequences of prior art transmissions--except the hydrostatic types, with their stated disadvantages--is the inexorable linkage between their kinetic and kinematic parameters, i.e., between the forces (or torques) generated and the speed relationships between the input and output shafts of the transmission. This linkage allows torque transfer from an engine (the driving member) to a load (the driven member) only in the same direction as the direction of the rotational speed of the load, i.e. output torque and output speed have the same mathematical sense, and the conversion ratio of the transmission is said to be positive under such circumstances. Thus, for example, it has heretofore not been possible to use engine torque applied in a direction opposite to the direction of rotation of the output shaft of the transmission, i.e. create a negative conversion ratio, to thereby allow true dynamic braking.
It is therefore yet another object of this invention to provide a transmission with a conversion ratio which can range from minus "a" to plus "b" where "a" and "b" can be arbitrarily chosen values, thereby allowing the transmission to apply torque both with--and against--the direction of the rotation of the load.